DNA Paternity Result Interpretation

Paternity tests results are based on the interpretation and analysis of 16 genetic loci (the location of a gene on DNA sequence). These loci are individually analyzed and combined together in order to obtain what is known as a combined paternity index (CPI). 

How does the Combined Paternity Index work?

The combined paternity index gives you a test result that is conclusive and accurate. Results will show that the alleged father of the child is either ‘excluded’ (the probability of not being the biological father) or ‘not excluded’ (the probability of being the biological father). 

An individual CPI is calculated against the general population based on commonalities of genetic information found in your analysis. From the CPI we may obtain results in percentages, such as, for example a 99.99%+ probability of paternity; easyDNA quotes both the CPI and the Probability of Paternity as part of your DNA paternity results as both these criteria are extremely important for your DNA test as well as for showing transparency in the way we carry out our DNA analyses. 

The DNA test report

Now let’s take a more detailed look into the DNA test report. When swabs are sent to easyDNA, they are analyzed and the DNA obtained from them are used to locate certain sites of chromosomes. These sites are called loci (locus in singular) and each one is associated with different sized fragments called alleles, which vary in length from one person to another. 

For each of the loci in the DNA test report, two numbers are presented, or in some cases when the alleles are identical, one number is presented. These numbers represent the size of the allele, which are then matched between the people involved in the DNA testing. The DNA test report will show each of the 16 loci involved in the test, which is to be found on the left hand column of the test. 

Individuals have two copies of chromosomes given to them by each of their parents.  For example, if a child has two alleles labeled as 1 and 2, and his or her mother has two alleles labeled as 1 and 3, then 1 is inherited from the mother supposing that 2 must be inherited from the father.

In order to assume that the alleged father being tested is the biological father, the DNA tests show all alleles matching. A comparison process is made between the child and alleged father whereby each locus is tested and matched. If the alleged father allele does not match the child at every locus then the alleged father is ‘excluded’ from the probability of being the biological father (it is worth adding that one or two exclusions can still indicate that the tested man is the biological father and in very seldom cases we do have exclusions in DNA tests. When this does happen, we carry out further testing, either by testing additional genetic markers or by requesting that the mother’s sample be analyzed as well for the paternity analysis).

Probability in this case refers to the ‘probability of relationship’ or in the case of paternity testing, the “probability of paternity”, which is the percentage likelihood for example 99.99%+ probability that the alleged father is the biological father and 0% if he is not the biological father. These probability percentages are also found in the paternity analysis report.